Here are some thoughts on speed training adapted from a talk I gave recently to a general audience.
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Do not run intervals too fast.

Running intervals too fast is not only possible, but is by far the most common mistake made in doing speedwork. My training logs show that I was running 400-metre repeats in around 78 seconds, about 3:10 per kilometer, when I ran a 5K race in 19:36 in the spring. When I switched to running longer repeats at a much slower pace, I went on to run 18:29 in the summer. An interval training session stresses a runner’s maximal aerobic capacity (what is known as VO2 max). This intensity can be maintained for around 10 minutes, or roughly 3K. Intervals run at 95 percent to 100 percent of this intensity help to increase aerobic capacity. Running faster than this is to run anaerobically, which will not produce the same benefits.

Do run longer intervals. I have not run 400-metre repeats since that spring over a year ago. It takes two minutes to reach VO2 max and though that time shortens with subsequent intervals, the stimulus will not be the same as longer intervals in which a greater time is spent at the proper intensity. This is why a workout of 20 x 400 metres in 90 seconds is not the same as 8 x 1 K in 3:45, even though they are at the same pace. The second workout results in much more time being spent at VO2 max, which is the goal of both workouts. The ideal duration of intervals is between two and six minutes, which could mean, depending on fitness, an interval length of anywhere from 300 metres to 2K.

Do run more volume. Instead of running the same or fewer number of intervals at a higher intensity, run a higher number of intervals or a larger overall volume at the same speed. For example, if a workout of 6 x 800 metres in 3:00 begins to feel easy, run 8 repeats instead of running 6 in 2:55. You will be spending more time at the appropriate intensity. Another method of increasing time spent at the appropriate intensity is to reduce the rest. The rest for an interval session should be a jog or rest of that is between half as long or just as long as the interval. The rest for 3 minutes of hard running should be between 90 seconds and 3 minutes.

Do not run intervals year-round. Intervals are a great way of getting into race shape, but they detract from the development of general aerobic endurance. The off-season, usually the winter, is a great time to improve your general fitness through a higher volume and moderately intense workouts such as tempo runs.

Do not run tempos too fast. A tempo run at its most basic is run at your lactate threshold, the point at which lactate is produced faster than it can be removed, accumulating in the blood. This intensity, which is best described as “comfortably hard,” can be maintained for roughly an hour. The simplest way to improve the speed at which this happens, a crucial determinant of performance in events ranging from 5k to the marathon, is to run for 20 minutes at this pace or for 40 minutes at a slightly slower pace. Running significantly faster than threshold pace is the equivalent of running a time trial and does not produce the physiological adaptations that were sought. It is not necessary to do this workout on a fixed course, though it is very helpful initially.

Do be creative and enjoy speedwork. I only run half of my intervals or tempos on a track or measured course. When the proper intensity for a workout becomes familiar, there is no need to run all workouts on a track. I find that running intervals off the track is a great way to incorporate hills into interval training. A change of scenery often allows runners to relax given that a track sometimes creates negative associations which can override focus. Running tempos on any closed course or loop is sufficient. While running the same workout allows progress to be measured, it is important to run a good mix of workouts to provide slightly different sorts of stimuli.

Do train specifically for a race and do understand the physiological demands of a race. Someone might train for a 3:00 marathon by running long runs at 9:00 per mile, easy runs at 8:00 per mile and half-mile repeats on the track at 6:00 per mile. This excludes the most crucial component of marathon training: time spent at marathon pace. Such a runner would be well-advised to replace intervals with a weekly run at marathon pace, which is around 7:00 per mile. Training for a half marathon requires a great deal of training at or slower than threshold pace. Training for a 10K race is a tricky blend of a high VO2 max and a high lactate threshold, but also intervals at 10K pace to create familiarity and pacing. Race pace for a 5K race is slightly slower than VO2 max pace, but also requires a high lactate threshold. It should also be noted that your fitness levels might change these values. For example, someone running a 10K race in an hour would be running at threshold pace, but someone who runs the distance in 35 minutes is in between VO2 max and threshold pace. Training for the course on which you expect to race is also important, of course. A tempo run that mimics the general layout of the course (e.g. early uphills, flat middle, downhill finish) is very helpful in this regard.

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People use “intervals” to mean all sorts of things. You are right that they are VO2max runs, and should take at least 2 minutes per. We try to do them, per Jack Daniels, for from 3 to 5 minutes each.

The other track workout (besides intervals and tempo runs) are repeats, which are shorter and faster than intervals and get full recovery. We use these in the early season for working on form, and every now and then to break up too steady a diet of intervals. They are the fun track workout because they’re fast and you get full recovery.

For each type of run, though, people should be aware of their target pace, and it varies among the various types of workouts. There are charts available that use recent race results to get target paces. One that uses Daniels’s calculations is http://runworks.com/calculator.html